Another soil scientist’s view on regenerative rhetoric. By Andrew Swallow.
Farmers experimenting with regenerative agriculture may find new systems and benefits, but beware associated sales pitches, unsubstantiated claims, and rhetoric rubbishing established practice that’s been proven by sound science, Lincoln University Professor of Biogeochemistry, Leo Condron says.
“My problem with regenerative agriculture is it is being advanced by painting a picture of a crisis that doesn’t exist in my opinion,” he told Country-Wide having read the report on p72 of this issue from one of Jono Frew and Peter Barrett’s Regenerative Agriculture Roadshows.
“It is being promoted as if there’s a massive systemic problem that we’ve got to fix, which I don’t agree with at all and it’s certainly not a crisis.”
That said, Condron adds that it is widely acknowledged that there are ongoing and emerging issues associated with use and management of soil in New Zealand that do need to be addressed, including erosion of hill country, loss of high quality land to urban development, conservation and protection of intensively managed lowland soils, and reduction and control of nutrient transfer from land to water.
Some practices promoted as regenerative are simply existing knowledge repackaged, such as use of minimum tillage to reduce loss of moisture, organic matter, and soil structure, he notes. The possible advantages of others, such as using highly diverse forage mixes, are unproven, while the claimed benefits of establishing and maintaining certain cation ratios have no sound scientific basis.
Cation exchange capacity is the ability of soil to retain and release cations and is a useful measure of soil fertility, but the main driver for that, and many other soil properties and processes, is organic matter, not cation ratios, he says.
Consequently, adequate soil organic matter is absolutely imperative for healthy, fertile soil, and many of the anecdotal benefits of regenerative agriculture relate to the adoption of practices designed to increase soil organic matter content. But increasing organic matter content of agricultural soil without drastically changing land-use is challenging, as is accurate measurement of changes in soil organic matter content over time.
With regard to soil biology, bacteria and fungi account for over 95% of soil organisms and their activities are governed by the supply of energy in the form of organic carbon from plant photosynthesis, which in turn is determined by the quantity and quality of organic matter inputs to soil.
There is no evidence that nitrogen fertilisers ‘make soil bacteria go nuts’, as Frew put it, and even if they did, that wouldn’t necessarily be a bad thing. It would simply be a consequence of more vigorous plant growth releasing more carbon into the soil in plant matter and root exudates, and probably via urine and dung too, resulting in a more abundant soil biology, including bacteria.
As for superphosphate ‘nuking’ soil biota, while there is some damage in the immediate vicinity of a fertiliser granule during dissolution, the scale of any effect needs to be taken into account.
Applied at 100-200kg/ha, superphosphate granules contact a minute proportion of the 750 to 1000t/ha of topsoil there is in the top 7.5cm and any damage caused to bacteria, fungi or other soil organisms will be minor compared to benefits from subsequent enhanced plant growth.
Research has shown that annual applications of up to 376kg/ha of superphosphate to a grazed pasture for over 60 years significantly enhanced soil biological activity, Condron says.
Scale is also key to understanding the likely impact of adding specific bacteria or fungi to soil as so-called “bio-stimulants”. Given that a hectare of soil may contain up to 15 tonnes of organisms including more than 100,000 different species of bacteria and fungi, Condron says it is difficult to envisage that the application of small quantities of bacteria or fungi will have any effect on the biological cycling and availability of nutrients. In fact, extensive research has shown that the application of commercial preparations of “plant growth promoting” and “phosphate-solubilising” bacteria have no significant impact on plant growth under field conditions.
Mycorrhizal fungi do play an important role in plant nutrition but many species are already present in agricultural soils and while introducing new species may be beneficial for some novel plants, this is usually done via inoculation of seeds rather than by field application, he notes.
Condron also questions whether humic acid or humate applications can make any difference to biological activity on a field scale. Humic substances, including humic acid and fulvic acid, make up over half the organic matter in soils naturally, hence a typical New Zealand topsoil of 750-1000t/ha at 10% organic matter already contains up to 50t of humic substances per hectare. Accordingly, it is very difficult to understand how applying small quantities of humate or humic acid preparations can make any significant difference to properties and processes associated with naturally occurring humic substances, and that’s backed up by the limited field research reported to date, he adds.
Limitations of soil tests is one area where Condron echoes Frew to some extent. Tests mainly reflect how soil chemical properties and processes influence nutrient solubility, which is appropriate for calcium, magnesium, and potassium where availability of the nutrient to plants is principally controlled by chemistry, he explains. However, more than 90% of the nitrogen and sulphur in soil is chemically bonded to carbon in organic matter (“organic nitrogen”, “organic sulphur”) and not immediately available to plants. Similarly, about 50% of total phosphorus is bonded to carbon (“organic phosphorus”).
Research has shown that annual applications of up to 376kg/ha of superphosphate to a grazed pasture for over 60 years significantly enhanced soil biological activity.
These nutrients are released or “mineralised” from organic matter by bacterial, fungal and plant enzymes cleaving the carbon-nutrient bonds, hence Olsen P tests and sulphate-S tests do not adequately reflect the role of soil biological processes in determining potential plant availability, though a soil test using an extended incubation period is available, at a price, to measure the potential for a soil to release nitrogen from organic matter.
Condron notes that reducing or eliminating mineral fertilisers appears to be a common strategy in regenerative agriculture but continued nutrient input is necessary in any sustainable agricultural system to replace nutrients removed in produce, he stresses.
That said, improving nutrient use efficiency and tightening nutrient cycles in agricultural systems is an ongoing objective of vast amounts of research worldwide in order to minimise environmental impacts and conserve finite non-renewable resources, especially phosphate rock.
“Overall utilisation of fertiliser nutrients in agriculture is very low. For example, only 10-30% of the phosphorus in fertiliser will be utilised by plants in the growing season following application.”
Most of it accumulates in the soil as stable forms of mineral phosphate and organic phosphorus, so-called “legacy P”. Similarly there are accumulated reserves of organic nitrogen and sulphur in many soils.
Research to access those reserves mainly focuses on using existing agricultural and novel plant species, plus their associated microorganisms (mycorrhizae etc), in various crop rotations and grassland systems, including intercropping, use of cover crops, and/or green manures.
Hence it is possible that increasing the diversity of plants in grazed pasture systems, as the regenerative advocates suggest, may enhance mobilisation of legacy soil P and reduce maintenance phosphorus fertiliser requirements, he says.
Whatever the drivers, scientifically sound, independent research into claims made by proponents of regenerative agriculture is desperately needed.
“We need hard, empirical data on what are the upsides and downside of these approaches,” he concludes.
